I think that the gradual slowdown is more likely to be a software rather than a hardware problem.
If you have netstat on your XP PC you can run that with -e to show layer 3 stats.
Cold start your PC, run
netstat -e -t 30 > C:\netstat.txt (this will run netstat every 30 seconds and pipe the output to the text file) and operate normally until you start to see the sort of slowdown problems you mention.
Stop netstat and look at the text file to see if the number of discards or errors has increased over the duration of your connection.
If it has, then the NIC could be at fault (not necessarily, but possibly).
Also check the event logs - system and application - to see if there's any errors or warnings concerning the network.
There is also the free MS Network Monitor available to you - you would need to download and install it and read up about how to use it to capture network statistics - there are MS troubleshooting KBs such as
Advanced network adapter troubleshooting for Windows workstations with the steps outlined.
SD